Field of the Invention
The present invention relates to an image sensor and an image capturing apparatus that uses the image sensor.
Description of the Related Art
Image sensors such as CCD image sensors and CMOS image sensors are mounted in image capturing apparatuses such as digital still cameras and digital video cameras. These image sensors have a large number of pixels that include photoelectric conversion units such as photodiodes that perform photoelectric conversion.
Generally, an imaging optical system is needed in order to form an image of a subject on an image sensing plane of the image sensor. A lens is used for the imaging optical system, and a subject image is formed on the image sensor by this lens. Here, a difference occurs in the light amount that is incident on pixels in a central portion and a peripheral portion of the image sensing plane, due to a difference in the angle of incidence of light between the central portion and the peripheral portion of the image sensing plane. This causes degradation in image quality such as decreased light at the periphery, and is particularly marked in the case where the distance between the lens and the image sensor (exit pupil distance) is short. Also, this difference in the light amount increases according to the distance from the central portion, and the decrease in light appears markedly particularly in the peripheral portion of the image.
FIG. 1 is a diagram showing an example of the amount of decrease in light at the periphery for a given lens position with a conventional flat image sensor. The horizontal axis shows the distance from the center position of the image sensor (diagonal position). Also, the vertical axis shows the degree to which light decreases, taking the brightness at the center of the image sensor as 100%. As seen in FIG. 1, the degree to which light decreases becomes greater as the distance from the center position increases.
With regard to this problem, Japanese Patent Laid-Open No. 2001-156278 proposes a method of curving the image sensor into a cylindrical shape or a spherical shape. With this technology, the distance from the center of the lens to each pixel can be kept constant, thus enabling degradation of image quality due to the decrease in light at the periphery to be reduced.
However, with the method disclosed in Japanese Patent Laid-Open No. 2001-156278, the entire image sensing plane of the image sensor is formed into a curved shape, and the overall thickness of the image sensor including the package increases. When the thickness of the image sensor increases, there is a problem in that the outer dimensions of an image capturing apparatus such as a camera or the like equipped with the image sensor also increase.
Furthermore, in an image capturing apparatus provided with a focal-plane shutter, the distance between the shutter and the image sensor lengthens at the central portion of the image sensor due to the image sensor having a curved shape. Thus, there is also a problem in that shutter efficiency drops, and precise exposure control of the image sensor can no longer be performed.